Both geopositive and geonegative behavior have been found to be sex-linked in Drosophila pseudoobscura. The same genes are important in giving geopositive behavior in each of three populations of this species. Geonegativity also appears to be genetically homogeneous in the three populations tested. The classification mazes have been successfully employed to define previously unknown behavioral effects of mutants of D. melanogaster which were originally isolated because of their abnormal countercurrent behavior but which have normal electroretinograms. Results of these tests are suggestive of the possible processes affected by countercurrent defective mutants. Phototaxis mazes are being used to select strains of D. melanogaster which prefer particular wavelengths of light. The amount of genetic variation for phototactic behavior available in single inseminated D. melanogaster females is being investigated by comparing the response to selection for positive and negative phototaxis in isofemale, isogenic, and highly variable strains. BIBLIOGRAPHIC REFERENCE: Woolf, C.M., Sasmor, H. and T. Markow. 1977. Positive and negative geotactic behavior: sex-linked traits in D. pseudoobscura. Behavior Genetics (in press).